350 is the most important number in the world—it's what scientists say is the safe upper limit for carbon dioxide in the atmosphere.

Two years ago, after leading climatologists observed rapid ice melt in the Arctic and other frightening signs of climate change, they issued a series of studies showing that the planet faced both human and natural disaster if atmospheric concentrations of CO2 remained above 350 parts per million.

Everyone from Al Gore to the U.N.’s top climate scientist has now embraced this goal as necessary for stabilizing the planet and preventing complete disaster. Now the trick is getting our leaders to pay attention and craft policies that will put the world on track to get to 350.
Is 350 scientifically possible?

Right now, mostly because we’ve burned so much fossil fuel, the atmospheric concentration of CO2 is 390 ppm—that’s way too high, and it’s why ice is melting, drought is spreading, forests are dying. To bring that number down, the first task is to stop putting more carbon into the atmosphere. That means a very fast transition to sun and wind and other renewable forms of power. If we can stop pouring more carbon into the atmosphere, then forests and oceans will slowly suck some of it out of the air and return us to safe levels.
Is 350 politically possible?

It’s very hard. It means switching off fossil fuel much more quickly than governments and corporations have been planning. But we can change that--if we mobilize the world to swift and bold climate action, and shift the world to a clean energy future.